LABORATORY and PLANT MEASUREMENTS on ANODE PROCESS in ALUMINUM ELECTROLYSIS

Transcription

1 LABORATORY and PLANT MEASUREMENTS on ANODE PROCESS in ALUMINUM ELECTROLYSIS Prepared by: Dr. János Horváth Doctoral School in Chemical Engineering and Material Sciences University of Pannon Veszprém

2 I. Introduction and Objectives This work is based upon principles of physical-chemistry and electrochemical technologies. Based upon these principles the aluminum electrolysis was considered as a process during which chemical and electrochemical reactions take place in molten salts, that would determine electrolytic process and its efficiency. The research had two main areas of focus. First is to measure the operational data in order to help to the technological development of aluminum smelters, second, to carry out laboratory measurements to understand the fundamental and electrochemical processes in aluminum electrolysis. The Research Institute for Non Ferrous Metals (FKI) and later the Aluminum Research Development and Prime Contracting Centre (Aluterv-FKI) was the area of high-quality research on physical-chemistry of cryolite-alumina melts. In parallel, laboratory measurements were also being conducted on how to decrease energy consumption and to increase the current efficiency. The industrial production of aluminum is based upon the Hall Heroult process which based on the electrochemical reduction of alumina in cryolite melt at 960 ο C. Due to the significant enthalpy associated with alumina generation the electrolysis process is extremely energyintensive. The main objective in this area has to include the reduction of energy consumption. There are two ways to reduce the energy consumption of electrolysis process, first is to increase current efficiency, second is to decrease cell voltage. Fundamental analyzing of current efficiency indicated that with sole laboratory measurement, due to other secondary reactions, the current efficiency could not be determined. The task was given to study the chemical processes in laboratory electrolysis cells while keeping in mind the practical requirements. It was obvious from the start that electrochemical methods will be suitable on test. The reason of this method is that the sign is directly coming from the cryolite-alumina melts. The difficulty was that there were not materials, which could withstand to the agressive cryolite-alumina melts at ο C. The boron nitride material was found indicating proper resistivity against fluoride melt and it had remarkable insulating characteristics at such high temperature. 2

3 Different laboratory cells and measurement methods were prepared with the aim to determine the current efficiency during laboratory electrolysis process. Even though many different types of cells were tried, but due to the secondary reactions, the results on current efficiency were not comparable with the industrial operational experiences. Thus the effort of research work was to set up a new current efficiency model by using the secondary reactions which cause decrease in current efficiency. During the first step polarization curves were recorded on graphite anode by potential sweep method. At the same time fundamental processes related to current efficiency were investigated. The electrochemical components (the extrapolated cell voltage) make up 40-45% of the total cell voltage components and their relationship with other parameters in the electrolysis process is of high theoretical and practical importance. Therefore unfortunately currently there is not a method to measure electrochemical components. The relevant literature does not contain results about electrochemical components of the electrolysis processes related to operational parameters of electrolysis cells. The electrolysis process control is based on created resistance measurements. The voltage component of the electrochemical reaction in electrolysis cells consists of equilibrium potential and anode-cathode overvoltages, considered to be constant without reference to other electrolysis parameters. This voltage component is constant and lies between 1,55-1,75V which is adjusted in different operations. The electrochemical components were measured in our laboratory test as well, their values could be varied between 1,18V-2,56V. Therefore it was reasonable to suppose that such a significant variation would also be evident in the values of extrapolated voltage in cell operation. Such significant changes in voltage have to be in a direct relationship with other parameters of the electrolysis process. 3

4 II. Experimental and Test Methods Study of processes on current efficiency In laboratory measurements three relevant reactions were identified which cause losses in current efficiency. 1) Experimental cell was made and analytical methods were developed to measure the solubility of dissolved aluminum and sodium in cryolite-alumina melts. Dissolved metals (aluminum and sodium) in cryolite-alumina melts and their reaction with CO 2 gas was investigated at plant electrolyte temperature and composition. The reaction of dissolved metals (aluminum, sodium) with CO 2 gas determined the rate of reoxidation process giving a component in current efficiency losses. 2) Depolarization effect of dissolved aluminum existing in ionic form in the anode process was determined by current interruption technique. The anode current density of this secondary electrochemical process taking place on anode, was determined by using literature activity data of aluminum (I)mono fluoride species. 3) In earlier cathode process tests it was stated, that the accumulation of sodiumfluoride in the molten aluminum/electrolyte boundary layer causes decrease in current efficiency in cathode process. The primary process - reduction of aluminum-containing species - takes place through this boundary layer. Here takes place the secondary process, too - the transfer of sodium - into the molten aluminum. The losses in current efficiency were described by the results of three different laboratory measurements and equations. These laboratory measurements were further complemented by measurements performed as part of a joint, collaborative effort with universities. 4

5 Measurements of electrochemical components During preparation of control of electrolysis process analysis all electrical parameters of cells were made. This gave the opportunity to perform large number of plant measurements in order to ascertain the characteristics of line current and cell voltage in operating aluminum electrolysis cells and to determine the extrapolated voltage at I=0 ka. Using the 16-Channel-Data-Collection and Processing System developed by Aluterv-FKI measurements were carried out on various cell designs and cell operations in order to determine the extrapolated voltages. The preliminary measurements indicated that it is not necessary to record the whole characteristic. It was established that 40-45% decrease of line current in four five steps the extrapolated cell voltage can be determined. It was demonstrated by determination of extrapolated voltage, that operational problems (alumina feeding control, anode spike) can be detected, current efficiency can be estimated and stable range of cell operation can be determined. Results indicated that the calculated electromotive force (EMF) was always higher than the measured extrapolated voltage at 3-8% alumina content range in the electrolyte. To explain this difference, results of laboratory measurements were examined. The main reason of difference was the depolarization effect of dissolved metal on anode. The variation of cell resistance was also measured as a function of time. The reason was investigated why for small changes in line current the cell resistance did not follow Ohm s law. 5

6 III. Thesis Points Achieved Results on Process Study of Current Efficiency 1.) The dissolved metal in cryolite-alumina melts consists of two components: sodium and aluminum and for their separation an analytical method was developed. The new sampling technique and new analytical method maintained the determination of their solubility in cryolitealumina melts and it was the base of the determination of the reoxidation process causing decrease in current efficiency. 2.) On the base of the study of reoxidation process parameters, interaction between dissolved metals and CO 2 gas was investigated at plant temperature in different electrolyte compositions. It was found that the diffusion process through the boundary layer was the rate-determining step when the convection effect was small and the inlet of CO 2 gas was far from the aluminum surface. In the case, when CO 2 gas disturbs the aluminum/electrolyte surface due to the extended convection, both sodium and aluminum content increased in the electrolyte and the rate of the reoxidation process increased as well. Relationship was determined among electrolyte composition, the electrolyte temperature and the anode-cathode distance in industrial applied parameters. The rate of the reoxidation process was also determined in the decrease of the current efficiency by equation. This process is considered as an atomic solution and solubility at given temperature and electrolyte composition is determined by convection transport of gases. 3.) The depolarization effect of dissolved aluminum existing in ionic form was measured by current-interruption-technique. Anode current density of secondary electrochemical process taking place on the anode was determined using literature activity data of aluminummonofluoride. According to the knowledge of the anode current density the rate of dissolved aluminum - taking part in the anode secondary reaction was determined onto the decrease of current efficiency. 6

7 4.) It was stated that the accumulation of sodium-fluoride in aluminum/electrolyte boundary layer causes decrease in current efficiency of cathode process. Primary process - the reduction of the aluminum containing components - takes place through this boundary layer. Here takes place the secondary process too the decomposition of the sodium causing decrease in current efficiency. Due to the research work of Solli the equation of the secondary process taking place on the cathode is known, so my own results onto the sodium-concentration data were used to describe the process causing decrease of current efficiency in the cathode process. Achieved results in Determination of the Electrochemical Components 1.) A measurement method was worked out to determine the extrapolated voltage at I=0 ka using the characteristic of line current and cell voltage in aluminum electrolysis cells. 2.) It was verified, that instead of the assumption of constant electrochemical components the calculated electromotive force (EMF) is practical to use in the process control of aluminum electrolysis cells. The calculation of electromotive force (EMF) is based on the parameters of the reduction cells. The components of the electromotive force are the Nernst potential and the overvoltages (anode reaction overvoltage, anode diffusion overvoltage and cathode overvoltage), nevertheless electromotive force (EMF) does not have any thermodynamic effect to the process. 3.) The extrapolated voltage varies in wide range between the value of 1,5V-2,5V at the normal operation. To these changes the operation parameters of the reduction cell were assigned. The diffusion overvoltage from the components of the extrapolated voltage increases in case when the alumina concentration is below 3% in the electrolyte. In this case the extrapolated voltage was higher then the calculated electromotive force caused by the increase of the anode diffusion overvoltage making difficult the extrapolation to I=0kA. 4.) The measured extrapolated voltages were in all cases less then the electromotive force calculated for the given reduction cells in the range of 3-8% alumina concentration. To the explanation of this difference served the depolarization effect of the dissolved aluminum among laboratory conditions, because this current-density-decreasing component influences the current efficiency loss. 7

8 5.) It was concluded that the measurement of the extrapolated voltage is suitable to characterise the operation. The measurements carried out in reduction cells with different cell constructions and different operation proved the relationship between current efficiency and extrapolated voltage. 6.) A method was worked out and applied to determine the stable range of the operation of the existing reduction cells by using the measurement of extrapolated voltage. 7.) It was concluded, that with a small change in the current line the change in cell resistance and time function - by using the non-ohmic nature of the cell resistance change can be used for different purposes: for the adjustment of the proper electromotive force (EMF), for the improvement of alumina feeding, for the decrease of the number of anod effects and for the early detection of abnormal cell operation. 8.) The relationship between current-efficiency-decreasing processes and extrapolated voltage measured in industrial electrolysis cells was stated. In the value of the extrapolated voltage appear processes occuring in the boundary layer. These processes can be influenced by the change of the boundary layer caused by convection. The effect of the secondary process taking place on the anode is the effect of depolarization of dissolved ionic aluminum. It occures in the value of extrapolated voltage and decreases it. This depolarization effect is the highest rate in the current efficiency loss. This loss completing with the losses of reoxidation process and cathode process a current efficiency indicator was determined which can be easily applied to characterize the electrolysis process. 8

9 IV. The Utilization of the Achieved Results in Practice 1) Laboratory measurements contribute to the determination of rate of the currentefficiency-decreasing processes appearing in the aluminum electrolysis cells. 2) Using the determination of the extrapolated voltage an opportunity opened to determine quickly the current efficiency. The achieved results were identified and used in the frame of international contracts to improve the smelter performance parameters. 3) The results achieved from the measurements and from the calculations were incorporated into models and expert s system. The measurement method is the organic part of the 16- Channel-Data-Collecting- and Processing System as the electrochemical knowledge of the process. 4) The knowledge received during the laboratory and industrial measurements was utilized in education, in international workshops and in university lectures. 9

10 V. List of Publications Related to Dissertation 1) J.Horvath: Laboratory and Plant Measurements on Anode Process in Aluminum Electrolysis Proceedings of 31 st International Symposium of ICSOBA, Krasnoyarsk, Russia, ) J.Horváth: Results of the Process Chemistry in Aluminum Electrolysis, Bányászati és Kohászati Lapok, , 1990 (in Hungarian) 3) R.N.Prasad, Gy.Várhegyi, J.Horváth: Behavior of Sodium and Calcium in the Process of Aluminum Electrolysis, Bányászati és Kohászati Lapok , 1979 (in Hungarian) 4) J.Ádám, J.Horváth: Analyzing of Voltage Fluctuations in Aluminum Electrolysis Cells. Laboratory and Plant Measurements,Bányászati és Kohászati Lapok, , 1973 (in Hungarian) 5) J.Horvath: Electrochemical Measurements for Determination of the Alumina Content in Aluminum Electrolysis, Dr. tech. Dissertation, University of Chemical Engineering, Veszprém, 1975 (in Hungarian) 6) M.Szakszon, E.Berecz, J.Horváth, G. Szina: Electrochemical Investigation of Aluminum Ion Dissolved in Alumina-Cryolite Melt Proceedings of the 4 th Conference on Applied Chemistry Unit and Processes, Vol.2, p.145,. Veszprem, Hungary, ) I.Ubrankovics, P.Káldi, J.Horváth, G.Szina: Transport Properties of the Reoxidation Reaction in Cryolite-Alumina- Carbondioxide System, Proceedings of the 4 th Conference on Applied Chemistry Unit and Processes, Vol.2, p.134, Veszprém, Hungary, ) L.Tikász, J.Horváth, J.Pőcze, M.Zaymus: Electrical Measurements as Technological Control Method for Aluminum Electrolysis Cells, Proceedings of the 4 th Conference on Applied Chemistry Unit and Processes, Vol.2, p.151, Veszprém, Hungary,

11 9) J.Horváth, J.Pőcze, L.Tikász, G.Szina: Experiments for Determination of Anode Overvoltage in Operating Aluminum Electrolysis Cells, Proceedings of the 4 th Conference on Applied Chemistry Unit and Processes, Vol.2, p.158, Veszprém, Hungary, ) J.Horváth, J.Pőcze, M.Zaymus: Determination of Electromotive Force (EMF) in Aluminum Electrolysis, Proceedings of 4 th Conference of the Socialist Countries on Molten Salt Chemistry and Electrochemistry, Balatonfüred, Hungary, ) J.Horváth: Anodic Overvoltage and Anode Effect on Carbon Anode in Cryolite-Alumina Melts, Proceedings of 4 th Conference of the Socialist Countries on Molten Salt Chemistry and Electrochemistry, Balatonfüred, Hungary, ) N.Sillinger, J.Horváth: Iron and Silicon Impurities in Aluminum Electrolysis, Light Metals 1990, p.369, Proceedings of Sessions 119 th Annual Meeting, Annaheim, USA, ) L.Tikász, J.Horváth, I.Horváth, M.Vajta: Computer Control System for Aluminum Electrolytic Cells: Model Studies and Plant Operation, Light Metals 1988,. Proceedings of Sessions117 th Annual Meeting, Phoenix, USA, ) J.Horváth, J.Pőcze, G.Szina, L.Tikász: Some Results of Research Works for Increasing the Energy Efficiency of Aluminum Electrolysis, International Symposium on Light Metals: Science and Technology, 37 th Annual Technical Meeting of Indian Institute of Metals, Varanasi, India, ) T.R.Ramachandran, J.Horvath: Plant Measurements for Energy Saving of Aluminum Electrolysis, Proceedings of 11 th International Symposium of ICSOBA on Quality Control in Aluminum Industry, p.323, Tapolca-Balatonfured, Hungary, ) T.R. Ramachandran, J. Horvath: Energy Savings in Aluminum Electrolysis, Journal of Electrochemical Society, Vol.44, Bangalore, India,

12 17) L.Tikasz, R.T.Bui, J. Horváth: Expert System-based Consulting Toolkit for Aluminum Electrolysis, Paper Presented at Conference of CIM, Canadian Institute of Mining Metallurgy and Petroleum, Vancouver, Canada, 1995 Lectures, education materials and brochures 1) Horváth J.: Outlook of the Development of the Aluminum Smelting Technology and the Globalization of the Aluminum Production, Scientific Session of Hungarian Academy of Science, Metallurgical Scientific Committee and Technical Committee of Academy at Veszprem Region, Building of Regional Academy, Veszprém, Sept, 2012 (in Hungarian) 2) Horváth J.: Aluminum Production in Electrolysis Process, Metallurgical Institute, Faculty of Materials Science and Engineering, University of Miskolc, (Supervisor: Prof.Dr. Tamás Török Head of Institute) Miskolc, Oct (in Hungarian) 3) Horváth J,: The Present Situation of the Aluminum Production Technology in the Word. International Committee for Study of Bauxites, Alumina and Aluminum (ICSOBA) Scientific Session, Hungarian Academy of Science, Institute of Materials and Enviromental Chemistry, Budapest, May (in Hungarian) 4) Horváth J,: Developing of Designs and Operation of Aluminum Reduction Cell Metallurgical Institute, Faculty of Materials Science and Engineering, University of Miskolc, (Supervisor: Prof. Dr.Tamás Török Head of Institute) Miskolc, Jan.31-Feb (in Hungarian) 5) Horváth J: Study of Electrochemical Processes in Aluminum Electrolysis and the Achieved Results in Aluterv-FKI, Scientific session, (Birth Centenary of Prof. Béla Lányi) OMBKE-ICSOBA, Aluminum Smelter at Inota, Inota, 1994 (in Hungarian) 6) J.Horváth, F.Schmidt: Energetic Appraisal and Possibility of Developments at Söderberg Cell (VSS)) Plenary Lecture, Jubilee Session of Aluminum Smelter at Tatabánya, Tatabánya, May (in Hungarian) 12

13 7) J.Horvath,: Aluminum Electrolysis, TALAT Course Material, edited, Al-together Phare program (editor: European Aluminum Association, teamleader: Dr.Eva Hídvégi) ) J.Horvath: Background of Current Efficiency on Aluminum Electrolysis, Report on visit at LECAP (Laboratoire experimental de controle avance des procedes) Universite du Quebec a Chicoutimi, Canada Aug ) J.Horváth: Current Trends in Aluminum Process Metallurgy, Paper Presented at UQAC- GRIPS, (Groupe de Recherche en Ingenierie des Procedes et Systemes) of Universite du Quebec, Chicoutimi, Canada, Sept ) J.Horváth: Theoretical Background for Improvement of Aluminum Electrolysis, Workshop Material, National Aluminum Company (NALCO ), Angul, India, July ) J.Horváth: Measurements and Data Evaluation for Improvements in Cell Operation. Workshop Material, National Aluminum Company (NALCO), Angul. India, July ) J.Horvath: Electrochemical Studies in the Hall-Heroult Process, Lecture at visit of Quebec University of Quebec a Chicoutimi, Chicoutimi, Canada, Aug ) J.Horváth, F.Mosóoczi, G.Szina, L.Tikász: Physical Chemistry and Practice of Aluminum Electrolysis, United Industrial Development Organization, Vienna,1984 Course Material to Group Training in China, Project No.: UC-UD-CPR ) L.Tikasz, J.Horváth, J.Pőcze, G.Szina, M.Zaymus: Electrical Measurements, Process Control and Electromagnetic Interactions in Aluminum Electrolysis, United Industrial Development Organization, Vienna, 1984 Course Material to Group Training in China, Project No.: UC-UD-CPR